Islamabad, Nov 11 (DPA): A huge bomb levelled a police building in Pakistan's largest city of Karachi Thursday, killing at least 20 people and injuring more than 50, rescue officials said.

The blast hit the headquarters of police's Crime Investigation Department (CID) located in the city's high security zone, a senior police official Iftikhar Tarar said.

"Unknown number of gunmen first exchanged fire with the policemen guarding the building and then a suicide bomber rammed a truck packed with explosives into the outer wall of the compound," Tarar said.

According to Tarar, around 15 people had so far been confirmed dead and over 100 were injured. "Most of those killed are policemen and the death toll might rise since dozens of people are trapped under the rubble."

Qamar Pervez, a spokesman of EDHI rescue service, said that they had moved 20 bodies and more than 50 injured to the hospital.

The blast damaged several nearby building and shops.

The investigation facility was located few hundred meters from important official buildings like the residences of the governor and chief minister for Sindh province of which Karachi is the capital.